The present invention relates to the thermal dissociation of chemical compounds, preferably allophanates, in particular compounds which bear a triply alkyl-substituted allophanate function as characteristic functional group (N,Nxe2x80x2,O-trialkyl allophanates).
N,Nxe2x80x2,O-trisubstituted allophanates formed by the addition of isocyanates onto urethane functions have been known for a long time and have been comprehensively described. They are frequently formed deliberately as crosslinking points in the production of polyurethanes or isocyanate-containing and isocyanate-capped prepolymers (e.g. in Houben/Weyl xe2x80x9cMethoden der Organischen Chemiexe2x80x9d in E. Muller Vol. XIV/2, Chapter III xe2x80x9cPolyurethanexe2x80x9d and D. Dieterich Vol. E20.2, Chapter II b y3) xe2x80x9cPolyurethanexe2x80x9dxe2x80x94Georg Thieme Verlag and D. Dieterich in xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d Vol. A21, Chapter xe2x80x9cPolyurethanesxe2x80x9dxe2x80x94VCH).
To determine the allophanate content of these polyurethanes or prepolymers, the allophanate groups have to be cleaved again. This allophanate cleavage can be carried out, for example, by aminolysis (cf., for example, D. Joel, G. Schulz, Plaste Kautsch., 32(4), 1985, 151-153; D. Joel, M. Weiler, Plaste Kautsch., 27(7), 1980, 374-376; M. Furukawa, T. Yokoyama, J. Polym. Sci.xe2x80x94Polym. Lett., 17, 1979, 175-180; I. G. Foliforowa, B. M. Bulgyin, L. I. Kopusov, Zavod. Lab., 41(6), 1975, 671-673).
Another method of determining the allophanate content is pyrolysis and subsequent chromatographic (GC) and/or spectroscopic analysis (IR, MS) of the dissociation products (cf., for example, N. Yoshitake, M. Furukawa, J. Anal. Appl. Pyrol., 33, 1995, 269-281; N. Yoshitake, N. Furukawa, T. Yokoyama, Polym. Degrad. and Stab, 29, 1990, 341-352 and 18, 1987, 341-348).
However, these studies relate exclusively to N,Nxe2x80x2-diaryl-O-alkyl allophanates as model compounds for polyurethane materials and elastomers. The pyrolysis is carried out at from 250xc2x0 C. to 550xc2x0 C. Apart from aryl isocyanates and the alcohol component, additional dissociation products such as carbodiimides and carbon dioxide are also formed in the pyrolysis.
A base-catalyzed thermal dissociation of allophanates to liberate isocyanates is described in DE 30 40 692 A1. However, this method is restricted to the dissociation of N,Nxe2x80x2-dialkyl-O-aryl allophanates and requires the presence of organic carbonates.
In the phosgene-free preparation of isocyanates by thermal dissociation of urethanes, there is often also undesirable formation of allophanates due to reaction of the urethanes used with the isocyanates liberated in the dissociation. These allophanates frequently form relatively nonvolatile bottom products, but can partly be converted back into the corresponding urethanes by alcoholysis in the presence of the corresponding 0-substituted carbamates at 240xc2x0 C. This procedure is described, for example, in DE 196 28 552 A.
The preparation of carbamic esters by alcoholysis of the corresponding allophanates is also described, for example, in DE 33 27 824 A1.
Furthermore, U.S. Pat. No. 3,392,184 discloses the thermal dissociation of trialkyl allophanates to give isocyanates. However, a disadvantage of this method is that the dissociation products obtained are stripped out of the reaction mixture in insufficient yield.
It is an object of the present invention to provide a process for the thermal dissociation of allophanates which avoids the above-described disadvantages of the prior art and makes it possible, in particular, to obtain the dissociation products in improved yields.
We have found that this object is achieved by a process for the thermal dissociation of allophanates in a reaction vessel, in which the gaseous dissociation products obtained are removed from the reaction vessel using an inert carrier gas or the vapor of a liquid as stripping medium and are condensed.
According to the present invention, temperature and pressure in the reaction vessel are selected so that the desired dissociation products are obtained in gaseous form.
The thermal allophanate dissociation is preferably carried out at from 150xc2x0 C. to 350xc2x0 C., in particular from 180xc2x0 C. to 280xc2x0 C. The reaction is preferably carried out under atmospheric pressure, particularly preferably under subatmospheric pressure. The pressure is very particularly preferably 0.01-1 bar.
The thermal allophanate dissociation can, if desired, be accelerated by means of suitable catalysts. As suitable allophanate dissociation catalysts, preference is given to using inorganic or organic metal compounds (e.g. metal alkoxides, acetylacetonates, carboxylates, halides and pseudohalides) of groups IIIa, IVa, Ib, IIb, IVb, VIb, VIIb, VIIIb, in particular compounds of aluminum, tin, copper, zinc, titanium, zirconium, molybdenum, manganese, iron, copper and nickel.
To separate the allophanate dissociation products from the reaction mixture, use is made, according to the present invention, of suitable inert carrier gases and/or vapors of liquids, nitrogen, inert solvent vapors and/or alcohol vapors as stripping medium
Preference is given to nitrogen, inert solvent vapors and alcohol vapors, very particularly preferably nitrogen.
According to the present invention, alkyl allophanates are preferably used as starting materials. The dissociation can be carried out using, in particular, the following compounds:
monoallophanates and/or polyallophanates derived from (cyclo)aliphatic monoisocyanates
monoallophanates and/or polyallophanates which are derived from (cyclo)aliphatic polyisocyanates and contain isocyanate groups and/or carbamate groups
monoallophanates and/or polyallophanates derived from (cyclo)aliphatic monoalcohols and/or polyalcohols.
Particular preference is given to using N,Nxe2x80x2,O-trialkyl allophanates.
The abovementioned alkyl allophanates can be dissociated on their own and/or in admixture with further allophanates and/or in admixture with high-boiling solvents which are nonvolatile and unreactive toward the dissociation products, in particular toward the isocyanates, under the chosen dissociation conditions.
It can be advisable, especially for the purpose of minimizing recombination and/or subsequent reactions (in particular allophanate and isocyanurate formation), to carry out the reaction in the presence of one or more high-boiling solvents, in particular solvents which are unreactive toward isocyanates.
As high-boiling solvents which are nonvolatile and unreactive toward isocyanates under the chosen dissociation conditions, it is possible to use, in particular, saturated long-chain linear and/or aromatic hydrocarbons, halogenated hydrocarbons and ethers. The solvents used are advantageously chosen so that the trialkyl allophanates used for the dissociation dissolve sufficiently in the respective solvent under the dissociation conditions selected.
It is also possible to deactivate reactive dissociation products, e.g. isocyanates, if necessary so as to prevent subsequent reactions. This can be achieved, for example, by chemical quenching (i.e. conversion into unreactive products) of the dissociation products, in particular the isocyanate, by means of suitable substances (e.g. monosubstituted or disubstituted amines, thiols, alcohols).
According to the present invention, preference is given to a process for obtaining isocyanate and urethane or isocyanate and alcohol by thermal dissociation of N,Nxe2x80x2,O-trialkyl allophanates. The substances mentioned can be formed by the following reactions:
Accordingly, a reaction carried out according to stage 1 can be employed for obtaining isocyanates and urethanes by thermal dissociation of the allophanates. Choice of the dissociation conditions (temperature and pressure) enables the allophanate dissociation to be stopped in a targeted manner at the first stage, viz. the formation of the corresponding N,O-dialkyl carbamates with elimination of the corresponding alkyl isocyanates.
Reaction temperature and pressure are preferably selected so that the dissociation products urethane and isocyanate are gaseous under the given conditions. Particular preference is given to reaction temperatures of 150-280xc2x0 C., very particularly preferably 180-260xc2x0 C.
The reaction is preferably carried out at a pressure of 0.01-1 bar, but particular preference is given to a pressure of 0.01-0.2 bar.
It is also possible to obtain isocyanates and alcohols by thermal dissociation of the allophanates according to stage 2. In this case, reaction temperature and pressure are chosen so that isocyanates and alcohol are obtained in gaseous form. The temperatures are preferably 180-350xc2x0 C., particularly preferably 220-300xc2x0 C.
Preference is given to carrying out the reaction at a pressure of 0.01-1 bar. Particular preference is given to a pressure of 0.02-1 bar, most preferably 0.04-1 bar.
The pyrolysis intermediate urethane remains in the reaction vessel and is dissociated further into isocyanates and alcohol.
Urethane dissociation product from stage 1 which has become entrained in the gaseous dissociation products can, if desired, be separated off by use of suitable separation apparatuses (e.g. packed columns or rectification columns) and returned as runback to the reaction vessel.
The invention is illustrated by the examples below: